From The School of Biomedical Sciences Wiki
Jump to: navigation, search

Meiosis is a type of cell division which the process that is characteristic of sexual reproduction occur only in eukaryotes[1]. It also can only occur in diploid cells, resulting in four unidentical haploid daughter cells. This contrasts with mitosis which can occur in both haploid and diploid cells, producing only two identical daughter cells. Therefore, meiosis results in a variation whereas mitosis produces exact copies of the parent cell. There are two stages of Meiosis, Meiosis I and II. This leads to the nucleus dividing twice but chromosome replication only occurring once. During prophase I, meiosis I involve recombination or crossing over of chromosomes[2]. Like mitosis, chromosomes in meiosis have duplicated in Interphase, during S phase and also ends with cytokinesis. Meiosis has 2 important purposes are its keep the number of chromosomes from doubling each generation and introduce genetic diversity in gametes[3]. In meiosis, if the chromosomes fail to segregate equally and correctly this can lead to genetic disorders. This is called non-disjunction. As a result of this, gametes contain the incorrect number of chromosomes and they are said to be aneuploid gametes[4].


Meiosis I[5].

Meiosis I is often referred to as the reductional phase, this is because the chromosome number is halved in meiosis 1. Meiosis 1 is separated into 4 stages. These are Prophase I, Metaphase I, Anaphase I and Telophase I.

Prophase I

In most higher organisms, prophase 1 can last several days. During prophase, I chromosomes pair, condense and crossing over occurs between non-sister chromatids. It is separated into 5 different stages. Similar to mitosis, centrioles move to opposite poles and spindle fibres start to form.


Leptotene, which is also known as leptonema is the first stage of Prophase I. During the Leptotene stage chromosomes coil and start to condense into long strands in the nucleus[6]. This is when the chromosomes first become visible. The threadlike chromosomes are called chromatonemata[7]. Then the two chromatids are joined together at the centromere. At this stage, the telomeres of the chromosomes are turned toward, and mostly attached to the same region of nuclear envelope[8].


During Zygotene the homologous chromosomes pair up and undergo synapsis where the synaptonemal complex between the homologous chromosomes start to form. Where a synapse has formed between homologous chromosomes, they are referred to as a bivalent. At the end of synapsis, the fused homologs will be double chromosomes but it looks like single chromosome under light microscope[9].


During the third substage of meiosis, the chromosomes continue to condense. Crossing over takes place at this stage and at each point of crossing over a chiasma is formed (singular: chiasma) between non-sister chromatids of homologous chromosomes. Each bivalent will have one chiasma, however, chromosomes that are longer will have at least three chiasmas.


The synaptonemal complex breaks down, allowing the synapsed chromosomes to separate. The chiasmata are now visible and are the point at which the chromosomes are still held together.


Diakinesis is the final substage of prophase 1. In this stage, the homologous chromosomes move further apart, however, non-sister chromatids are connected via the chiasmata. Terminalization occurs as the chiasmata move towards the ends of the tetrad. Toward the end of diakinesis, the nucleolus and nuclear envelope start to break down and spindle fibres begin to form in preparation for metaphase I.

Metaphase I

Bivalents or tetrads (four chromatids) align on on the metaphase plate (the equator of the cell) and spindle fibres attach to the kinetichores; protein structures located at the centromeres. In this stage, the nuclear envelope has been fully disintegrated.

Anaphase I

Disjunction occurs. This is when the mitotic spindles pull the tetrads apart to forming dyads, which migrate to opposite poles.

Telophase I

The nuclear envelope may form around the dyads and cytokinesis (cell division) occurs.


Interphase does not have to occur between Telophase I and Prophase II, but it can occur. Unlike traditional interphase, there is no DNA replication but growth and biosynthetic activities can still occur.

Meiosis II

As meiosis forms 4 gametes from a single cell a second round of cell division must take place, Meiosis II resembles mitosis more than meiosis I as it is a simple division with no crossing over. The process Meiosis I generates haploid cells (chromosome number already halved) therefore no DNA replication occurs between Meiosis I and II and the number of chromosomes remains unchanged throughout meiosis II.

Prophase II

Unlike Prophase I no chiasmata form and no crossing over occurs. If a nuclear envelope has formed during Telophase I, it is broken down. Centrioles move to opposing poles and spindle fibres start to form.

Metaphase II

The dyads align on the metaphase plate and spindle fibres attach to the kinetechores.

Anaphase II

The dyads are pulled apart by spindle fibres and the myads arrive at opposite poles.

Telophase II

The nuclear envelope reforms around the myads. Cytokenesis occurs and the cells divide. This leaves us with 4 non-identical daughter cells, also known as gametes[10][11].


  1.,1 December 2015 (cited by 2003) Schmidt, Silke, available from
  2.,1 December 2015 (cited by 2003) Schmidt, Silke, available from
  3. Daniel L.Hartl and Maryellen Ruvolo,(2012), Genetic, Analysis of genetic and genome, 8th edition, United States of America: Jones and Bartlett Learning.
  5. Hartl DL and Jones EW (2009) Genetics: Analysis of Genes and Genomes, Seventh Edition, USA, Jones and Bartlett Publishers
  10. Hartl DL and Ruvolo M (2012), Genetics: Analysis of genes and genomes, eighth edition, Jones and Bartlett learning
  11. Hartl DL and Ruvolo M (2012), Genetics: Analysis of genes and genomes, eighth edition, Jones and Bartlett learning
Personal tools